A conventional method for stabilizing oscillation frequency separation among a plurality of laser devices has been described in U.S. Pat. No. 4,835,782 issued on May 30, 1989.
In the method for stabilizing an oscillation frequency separation among a plurality of laser devices, a plurality of laser devices are controlled to emit output lights each having a predetermined frequency, so that frequency separation is stabilized. Reference pulses are produced in an optical resonator which has periodic resonant frequencies and receives a frequency swept signal, and beat signals are produced in accordance with the combination of the frequency swept signal and oscillation frequencies of the plurality of laser devices. The reference signals and beat signals thus produced are processed to produce error signals which are time differences between the producing times of the both signals. The plurality of laser devices are controlled to be driven, such that the error signals become a predetermined value.
According to the conventional method for stabilizing oscillation frequency separation among a plurality of laser devices, however, there is a disadvantage in that the number of laser devices which are simultaneously controlled in the oscillation frequency separation is limited to approximately 10, because the number depends on a sweeping range of the frequency of the reference laser device.